1. Field of the Invention
The present invention relates to a power supply system comprising capacitors that are switchably connected to decrease variations of an output voltage.
2. Description of the Related Art
An electric double-layer capacitor is physically charged and, hence, can be charged more quickly than chemical batteries, such as lead-acid batteries and nickel/cadmium batteries. However, a power supply system using electric double-layer capacitors produces a terminal voltage V that varies greatly given by equation Q=CV.sup.2 /2. An electrical energy storage system making use of the energy capacitor system (ECS) using electric double-layer capacitors has attracted attention as a power supply for an electric vehicle or as a large-scale electrical energy storage system.
The ECS has been introduced in various literature (e.g., Electronic Technology of Japan, 1994, 12, pp. 1-3; T.EEE Japan, in Japanese, B, Vol. 115, No. 5, 1995, pp. 504-610) as an electric energy storage system consisting of capacitors, parallel monitors, and current pumps. These capacitors are connected in series to form a capacitor bank. The parallel monitors are connected across the respective capacitors of the bank. When the capacitors are charged to more than a value set for the parallel monitors, these monitors bypass the charging current or discharge the capacitors until the terminal voltage reaches the set voltage. In this way, the state of charge of the capacitors is controlled. The parallel monitors may also be connected across the capacitor banks.
These parallel monitors act to make uniform the maximum voltages, prevent reverse currents, detect the terminal voltages on completion of charging, and perform controlled operation even if the capacitors have nonuniform characteristics and varying amounts of residual charge. Therefore, the parallel monitors are quite important in making almost full use of the capability of the capacitor bank to store electrical energy.
On the other hand, in a power supply system using capacitors whose terminal voltages vary widely at a high rate as energy is drawn from the fully charged state, it is required to suppress variations in the output voltage due to variations in the terminal voltages of the capacitors.
Therefore, a power supply system for switching capacitors between series connection and parallel connection to reduce variations in the voltage has been proposed as disclosed in U.S. Pat. No. 5,734,205. FIGS. 11(A), 11(B), and 11(C) show examples of the structure of a power supply system in which capacitors are switched between series connection and parallel connection. In this configuration, as the terminal voltage drops, the capacitors are switched from parallel connections to series connections.
A series-parallel switching circuit for capacitors C1 and C2 of this power supply system is shown in FIG. 11(A). The state is switched to a state shown in FIG. 11(B), where more stages are cascaded. In this way, the state is varied in a stepwise fashion according to the state of charge. Consequently, variations in voltage can be reduced further with increasing the number of stages.
Where the voltage variations are reduced by the method described above, a large number of stages are switched from parallel to series connection. As the number of stages increases, more switches are necessary. That is, as can be seen from FIG. 11(A), three switches Sp1, Sp2, and Ss1 are used in one stage and so three switches are required for each stage.
Furthermore, these switches are used in power applications. Hence, use of large-sized electromagnetic contactors or power semiconductor devices, such as giant transistors, IGBTS, GTOs, and thyristors, is necessary. Therefore, the number of components including driver circuits for the switches and radiators is increased. Also, a large space is necessary to mount them. As a result, the cost of the system is increased. Furthermore, the reliability of the switches poses problems.
In addition, when the connection is switched from parallel to series, if the voltages across the capacitors C1 and C2 are not uniform, a large crosscurrent flows between the capacitors C1 and C2. To prevent this crosscurrent, protective circuits A1, A2, and corresponding switching elements Q1-Q3 are necessary, as shown in FIG. 11(C).